An objective of this project is to study the effect of hyperthermia and radiation on experimental animal tumors, i.e., studies on the alteration of hypoxic cell fraction following hyperthermia, on the effect of pH on tissue response heat and on thermal sensitivity of various tumors. A spontaneous fibrosarcoma (FSa-II) transplanted into the C3Hf/Sed mouse foot is treated in a water bath. Assay methods include analysis of tumor growth delay, determination of treatment time to yield a tumor control in half the treated animals (TCD50) and determination of cell survival in vivo. Most significant findings are as follows: a. A large tumor is more sensitive than a small tumor to heat. Four different sized tumors were treated at 43.5 degrees centigrade. Tumor growth study indicated that the thermal sensitivity increased with increasing tumor size and the dose response curve was of single component. TCD50 values of 35, 110, 270 and 800 cubic mm tumors were 170, 141, 123 and 111 min at 43.5 degrees centigrade. b. The heat resistance developed in normal and tumor tissues, and the kinetics in both tissues was comparable. In general 1) the resistance developed within 24 hours and subsided gradually, 2) the magnitude of the resistance depended on the damage produced, and 3) the resistance developed after each of fractionated doses.